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By Keith Coffman and Daniel Wallis CENTENNIAL, Colo. (Reuters) - Colorado's long-awaited cinema massacre trial began on Monday with opening statements in which jurors were asked to decide whether gunman James Holmes was insane when he killed a dozen moviegoers in 2012, or a calculating mass murderer who deserves execution. Arapahoe County District Attorney George Brauchler painted a picture for jurors - 19 women and five men - of the cool night in July when 400 people filed into a cinema in suburban Aurora to be entertained, and someone came to "slaughter" them. Through this door are bullets, blood, brains and bodies," Brauchler said, showing images of the theatre where Holmes killed 12 people and wounded 70 at a crowded midnight screening of the Batman film "The Dark Knight Rises." "When we're done getting all these horrific details ... I'm going to come back and stand in front of you, right here, and I'm going to ask you to reject that guy's claim he didn't know right from wrong," he said, his voice breaking with emotion.